⚛️ quantum physics

Multiplexed ion-ion entanglement over $1.2$ kilometer fibers

This paper presents the first demonstration of multiplexing-enhanced heralded entanglement between two trapped-ion quantum nodes over 1.2 km of fiber, achieving a 4.59-fold speedup in generation rate and 95.9% fidelity through the use of 10 temporal photonic modes.

Original authors: Z. B. Cui, Z. Q. Wang, P. Y. Liu, Y. Wang, P. C. Lai, J. X. Shi, Y. D. Sun, Z. C. Tian, H. S. Sun, Y. B. Liang, B. X. Qi, Y. Y. Huang, Z. C. Zhou, Y. K. Wu, Y. Xu, Y. F. Pu, L. M. Duan

Published 2026-09-09

📖 4 min read🧠 Deep dive

CC BY 4.0

Original authors: Z. B. Cui, Z. Q. Wang, P. Y. Liu, Y. Wang, P. C. Lai, J. X. Shi, Y. D. Sun, Z. C. Tian, H. S. Sun, Y. B. Liang, B. X. Qi, Y. Y. Huang, Z. C. Zhou, Y. K. Wu, Y. Xu, Y. F. Pu, L. M. Duan

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The dream of a quantum internet relies on a single, fragile connection: the ability to link two distant machines so that they share a secret state, a phenomenon known as entanglement. Imagine two coins that, no matter how far apart they are, always land on opposite sides when flipped; in the quantum world, particles can be linked in this way, forming the backbone for future technologies like unhackable communication and powerful distributed computers. However, building this network is incredibly difficult because the signal carrying the connection weakens and gets lost as it travels through fiber optic cables, especially over long distances. To make these links work, scientists must create the connection quickly and reliably before the signal fades, a challenge that has long limited the scale of quantum experiments to small, local setups.

A team of researchers at Tsinghua University has now taken a significant step forward by demonstrating a method to speed up this process using a technique called multiplexing. They successfully created an entangled link between two trapped-ion quantum nodes separated by 1.2 kilometers of optical fiber. In their experiment, the scientists used calcium ions, which are single atoms held in place by electromagnetic fields, as the quantum nodes. Each node contained one ion, and the researchers employed a clever strategy to overcome the slow speed of light and the low probability of a successful connection. Instead of trying to create the link once and waiting to see if it worked, they sent out ten rapid, successive attempts in a single round. This approach effectively gave them ten chances to succeed where they previously had only one, significantly accelerating the rate at which the entanglement was established.

The setup involved two distinct locations, named Alice and Bob, situated 2 meters apart in the lab but connected to a central measurement station by 600 meters of fiber on each side, creating a total path of 1.2 kilometers. The process began by exciting the ions with laser pulses, causing them to emit photons, or particles of light. These photons traveled through the fibers to the center, where they were measured. If the detectors at the center saw a specific pattern of light, it signaled that the two distant ions had become entangled. Because the speed of light limits how fast information can travel back and forth, the researchers had to wait for the confirmation signal to return before knowing if the attempt was successful. By using ten time-based modes, or ten distinct time slots for these attempts, they managed to increase the speed of generating these connections by a factor of 4.59 compared to using a single attempt at a time.

The results were not just faster; they were also remarkably accurate. The team measured the quality of the entanglement and found a fidelity of 95.9 percent, meaning the connection was extremely close to the perfect theoretical state. This level of precision is notable because it was achieved over a distance much longer than previous records for this type of system, which had been limited to just a few hundred meters. The researchers also confirmed that the ions could hold onto this quantum state for hundreds of milliseconds, a duration long enough to perform further operations. This durability is crucial for scaling up the network, as it allows time for the system to process information and correct errors.

This work represents the first time such multiplexing has been used to enhance entanglement between trapped ions over a long fiber link. While previous experiments with other types of materials had shown similar speed-ups, this study proves the method works for ions, which are known for their high stability and precision. The researchers used a dual-type design for their nodes, separating the part of the ion used for communication from the part used for memory, a feature that will allow them to add more ions and qubits in the future without disrupting the system. By combining this architecture with the multiplexing technique, the team has created a robust building block for a larger quantum network. The findings suggest that with further refinements to reduce technical noise, such as laser instability or phase errors, the fidelity could potentially exceed 99 percent, bringing the vision of a large-scale, functional quantum internet one step closer to reality.

Technical Summary: Multiplexed Ion-Ion Entanglement over 1.2 Kilometer Fibers

Problem Statement
The realization of large-scale quantum networks and repeaters relies on the efficient and high-fidelity establishment of heralded entanglement between remote quantum nodes. A primary bottleneck in this process is the limited efficiency of generating matter-matter entanglement over long distances, particularly because the attempt rate is fundamentally constrained by the round-trip communication time ( $L/c$ ) in optical fibers. While multiplexing strategies have been proposed to accelerate entanglement distribution by increasing concurrent entangling attempts, previous experimental demonstrations of multiplexing-enhanced matter-matter entanglement were restricted to solid-state systems. Furthermore, existing records for matter-matter entanglement fidelity over fiber distances beyond the laboratory scale (exceeding 520 m) have been limited to approximately 90% across all physical platforms. Trapped-ion systems, despite holding records for high-fidelity gates and long coherence times, had not yet demonstrated multiplexed entanglement over extended fiber distances.

Methodology
The authors constructed a quantum network consisting of two remote nodes, Alice and Bob, each containing a single trapped $^{40}\text{Ca}^+$ ion. The nodes were separated by 2 meters physically but connected via optical fibers totaling 1.2 km (600 m per node to a central measurement station).

Dual-Type Encoding: The system utilizes a dual-type encoding scheme to separate the "communication qubit" subspace (involving $|S_{1/2}\rangle$ , $|D_{3/2}\rangle$ , and $|P_{1/2}\rangle$ states) from the "memory qubit" subspace (involving $|D_{5/2}\rangle$ ). This spectral isolation allows dissipative operations (cooling, pumping, entanglement excitation, and detection) to occur in the communication subspace without disturbing the stored quantum information in the memory subspace.
Ion-Photon Entanglement Generation: The process begins with optical pumping to initialize the ion. A picosecond 397 nm laser pulse excites the ion to the $|P_{1/2}\rangle$ state. The ion spontaneously decays to the $|D_{3/2}\rangle$ level with a low branching ratio of 6%, emitting an 866 nm photon. The polarization of this photon is entangled with the ion's state. Subsequent Raman pulses transfer the entanglement to the memory qubit subspace ( $|D_{5/2}\rangle$ ).
Multiplexing Scheme: To overcome the low branching ratio and the latency imposed by the long fiber, the authors employed temporal multiplexing. Instead of a single excitation attempt per cycle, they executed a block of 30 rounds, where each round consisted of 10 successive "excitation-pumping" cycles. This utilized 10 temporal photonic modes. Crucially, intermediate optical pumping was performed using only the 397 nm laser to preserve the population in the $|D_{3/2}\rangle$ level if a prior excitation was successful, thereby reusing the population to increase the effective branching ratio.
Heralding and Feedforward: Photons from both nodes were transmitted to a central station for a two-photon Bell-State Measurement (BSM). A microprocessor ("QNetWorker") analyzed detector clicks to identify the specific coincidence pattern corresponding to the Bell state $|\Psi^+\rangle$ . Upon detection, a heralding signal was sent back to the local nodes via the same fiber length to simulate realistic round-trip latency. The local controllers then applied Raman pulses to finalize the ion-ion entanglement based on the heralding signal.

Key Contributions and Results

First Multiplexed Trapped-Ion Entanglement: This work presents the first experimental realization of multiplexing-enhanced heralded entanglement between two trapped-ion quantum network nodes.
Distance Record: The experiment successfully established entanglement over 1.2 km of fiber, surpassing the previous trapped-ion record of 520 m.
Efficiency Enhancement: By multiplexing 10 temporal photonic modes, the system achieved a 4.59-fold speedup in ion-ion entanglement generation compared to a single-mode case. The enhancement factor was observed to nearly saturate at 10 modes due to the population dynamics of the $|S_{1/2}\rangle$ and $|D_{3/2}\rangle$ levels.
High Fidelity: The experiment achieved an ion-ion entanglement fidelity of 95.9 ± 1.5% over the 1.2 km distance. This sets a new record for heralded matter-matter entanglement fidelity at extended fiber lengths across all physical platforms and is competitive with laboratory-scale setups.
System Scalability: The authors demonstrated a dual-type architecture that, while using a single ion per node in this experiment, is inherently designed to support future scaling to multiple qubits per node and multi-node networks.

Significance
The paper establishes a critical building block for future large-scale quantum networks by demonstrating that multiplexing can effectively overcome the efficiency limitations of long-distance entanglement distribution in trapped-ion systems. The combination of high fidelity (95.9%) and significant speedup (4.59x) over a kilometer-scale link validates the viability of trapped ions for practical quantum repeaters. The authors note that the current infidelity is primarily attributed to technical imperfections (Raman detuning, phase errors, laser instability, etc.) rather than fundamental limits, suggesting that further optimization could push fidelities beyond 99%. The demonstrated architecture paves the way for integrating additional multiplexing schemes (such as ion shuttling and fiber arrays) and scaling to tens of kilometers via quantum frequency conversion, ultimately supporting advanced tasks like entanglement purification and fully functional quantum repeater nodes.

Technical Summary: Multiplexed Ion-Ion Entanglement over 1.2 Kilometer Fibers

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